1. Field of the Invention
The present invention relates generally to arc welding control systems, and relates more particularly to adaptive welding systems which sense the characteristics of an inverted seam or flange weld such as the edges of turbine blades and the like in order to adjust the center line of oscillation of the welding electrode and other welding parameters as the electrode is moved along the inverted seam.
2. Description of the Prior Art
Recent times have seen drastic improvements in adaptive arc welding systems which sense the position of a welding electrode relative to a joint being welded and guide the electrode along the seam during the welding process. Marked improvement in techniques for obtaining position information from the highly noisy arc voltage or current signal has led to attempts to develop control systems which sense the characteristics of the seam being welded by sensing and analyzing a parameter of the welding arc.
There is presently a particular need for an adaptive control system which successfully adapts to a classic edge joint for flange welds. An "edge joint" is in actuality an "inverted" seam and may be defined as a joint between the edges of two or more parallel or mainly parallel members. The weld on an edge joint is typically called a "flange weld", and requires that the welding electrode be moved longitudinally along the edge joint of workpiece while maintained substantially above the surface to be welded.
In addition to welding two members in a classic edge joint, it is also frequently desired to weld-overlay or hard-surface a single edge for the purpose of hard facing or building up in a remanufacturing process. For example, the remanufacturing of turbine blades and vanes by weld overlay allows re-use of the expensive blades and vanes. For purposes of the discussion which follows, the terms "edge joint", "flange weld", and "inverted seam" are used interchangeably and mean any workpiece having a convex lateral geometry.
A typical aircraft gas turbine engine can contain over 4,000 air foils, making the air foils or blades the most numerous component in the engine. Since the turbine engine's performance is directly related to the temperature under which it can operate, turbine blades and vanes are designed with high temperature operation as an objective. Progress in turbine blade materials such as the super alloys has moved from air melting, to vacuum melting, and then to directional solidification fabrication techniques. Progress in turbine vane materials has shown a similar trend with a move from investment case alloys to dispersion strengthened alloys.
In addition to these material considerations, turbine blades and vanes must be manufactured to very tight contour tolerances. Turbine blades and vanes vary considerably in dimension and typically include a twist around a longitudinal axis as great as four degrees. The blades and vanes with their compound curves, thin cross-sections, and lack of flat holding surfaces are difficult to fixture securely and hold to required tolerances during machining.
The net result of all of these design and manufacturing considerations is a high initial manufacturing cost. It is therefore desirable to remanufacture the worn blades and vanes rather than discard them and replace with new blades and vanes.
Prior to the present invention, it had been thought that techniques for adaptively sensing a workpiece through the welding arc itself required distinct features of the workpiece such as sidewalls to be present in order for the control system to adaptively track the welding electrode along the workpiece. Absence of such workpiece features in articles such as turbine blades and vanes led those involved in research and development in adaptive welding control to concentrate their efforts towards controlling welding for workpiece types possessing more readily sensable features.